In prior art, a crinkling device as disclosed in the preamble of specification EP 0 511 870 A1 for use with a wrapping machine to adjust the width of a film web to be wrapped around an object between a full web width and a narrowed, string-like web width is known.
The crinkling device comprises a frame secured to the film dispenser of the wrapping machine, said film dispenser carrying a film web roll of film web having a longitudinal upper edge and a longitudinal lower edge.
The crinkling device further comprises a first wheel rotatably mounted on the frame and a second wheel rotatably mounted on the frame at a distance from the first wheel.
An endless drive element is passed over the first wheel and the second wheel so that a first drive element portion and a second drive element portion are formed between the wheels, said portions extending adjacently and parallel to each other in the widthwise direction of the film web. Connected to the first drive element portion is a carriage guided in the frame so as to be movable in the widthwise direction of the film web.
A first crinkling element is fastened to the first carriage to crinkle the upper edge of the film web. The crinkling device further comprises a second carriage, which is also guided in the frame so as to be movable in the widthwise direction of the film web. A second crinkling element is mounted on the second carriage to crinkle the lower edge of the film web.
A power means, consisting of a pneumatic cylinder, is arranged to move the second carriage in the widthwise direction of the film web. Mounted on the same shaft with the upper first wheel is a third wheel of a smaller diameter. Near the lower second wheel is a fourth wheel rotatably mounted on the frame on a different shaft than the second wheel. An endless second drive element is passed over the third and fourth wheels. The second carriage is fastened to the second drive element. Because of the difference between the sizes of the first and second wheels, a transmission ratio is formed between the first drive element and the second drive element such that when the second carriage is moved by the pneumatic cylinder, this movement is transmitted with the aforesaid transmission ratio into a movement of the first carriage. When the wrapping film is to be crinkled, the first carriage (and the first crinkling element) moves downwards through a distance longer than the upwards movement of the second carriage (and the second crinkling element) past the level of the second carriage. Due to the above-described mechanical transmission, the movements of the first and second carriages take place simultaneously. The prior-art crinkling device has the advantage that it uses only one power means.
A problem with the crinkling device disclosed in the above-mentioned specification is that it always crinkles the film web from the upper and lower edges without allowing them to be crinkled separately and independently of each other. Moreover, the device has a complex construction as it comprises two drive elements and two pairs of sheaves. Therefore, the crinkling device also comprises a large number of maintenance objects and parts subject to wear and replacement. Due to the complex construction, the manufacturing costs of the crinkling device are high. A further problem is that the distance through which the first and second crinkling elements travel during the crinkling action can not be varied without replacing components, i.e. without changing-the transmission ratio between the running speeds of the first and second drive elements by replacing the third wheel with a wheel of a larger or smaller diameter.
Another prior-art crinkling device works on the drive screw/nut principle, using one power means to rotate the drive screw, which has been arranged to move a first drive nut connected to a first carriage with a first crinkling element attached to it and a second drive nut connected to a second carriage with a second crinkling element attached to it. When the drive screw is rotated, the two carriages and the respective crinkling elements move in opposite directions through equal distances, with the result that the film web is contracted into a string at about the middle of the device. The problem is that the travel of each crinkling device is always the same and it is not possible e.g. to produce a slighter crinkling at the lower edge of the film web and a greater crinkling at the upper edge so as to produce from the film web a string located near the level of the lower edge of the original full web width.